It is well known to provide propulsion units that are suspended below the hull of a vessel, typically a ship in order to provide the ship with propulsion, and such propulsion units are commonly referred to as PODs. The concept of a POD for ship propulsion has been known for some time (examples are shown in U.S. Pat. No. 5,403,216, and European Patent No. 1 010 614) and is now in common use. In such an arrangement, the propulsion motor, which is generally electrical, is contained in a pod-like housing suspended below the hull of the vessel. The motor is directly connected to one or more propellers at one end, or both ends, of the pod housing. In cases where there is a propeller at only one end, the propeller can be either in front of or behind the pod casing relative to the water flow.
It will be appreciated that as the ship moves, the POD suspended therebelow will experience drag, which will oppose the motion of the ship. There is therefore a desire to reduce the physical dimensions of the POD so as to minimize the drag experienced by the ship. Therefore, PODs generally have minimal access to the insides thereof, and the propulsion motor is generally mounted on, or in close proximity to the wall of the POD. Therefore, vibrations from the propulsion motor are readily transmitted through the wall of the POD, leading to noise being passed from the POD into the surrounding water.
In many circumstances, it is desirable to minimize the level of noise transmitted to the surrounding water. For instance, cruise ships may want to travel into environmentally sensitive areas, and environmental research vessels or fisheries research vessels, etc., may want to minimize noise at all times. However, it is a problem that known noise isolation systems tend to require an increase in the size of the POD, and that the design of the POD therefore tends to be a compromise between low noise and small size.